The present invention relates to methods and systems for analyzing particles in a dilute fluid sample, and more particularly to a flow cell for producing a thin stream of specimen fluid from which optical images thereof can be obtained.
Methods and systems for analyzing particles and particularly sediments are well known in the art, as disclosed in U.S. Pat. Nos. 4,338,024 and 4,393,466, which are incorporated herein by reference. Such systems utilize a flow cell though which fluid samples (specimen fluids) are passed, and a particle analyzer for capturing still frame images of the fluid passing through the flow cell. Thus, the flow cell positions and presents the sample fluid containing particles of interest for analysis. The more accurately that the sample fluid is positioned by the flow cell, the better the analysis of the particles therein that can be made.
Typical flow cells cause the sample fluid, and a sheath fluid that buffers the sample fluid, to flow together from a large entry chamber into a small cross section examination area or region. The transition from the inlet or entry chambers to the examination region forms a hydrodynamic lens that squeezes both the sample fluid and the sheath fluid proportionally into the smaller space. Where the particles of interest are microscopic particles, the resulting cross-section space occupied by the sample fluid must be positioned within the depth of field of the analyzer, such as an optical system or a laser system, to obtain the best analytical information. For the best hydrodynamic focus, a large area of sheath flow must envelop the small area of sample fluid without any swirling or vortices. Thus, uniform flow of sample and sheath fluids through the flow cell is essential for optimal operation of particle analyzers. The sample mass transfer characteristics should be reproducibly controlled, so that the specimen cross section is sufficiently wide (e.g. 1 mm wide) for the specific application and measurement technique, with a thickness commensurate with the requirements of the detection method, while the fluid velocity is slow enough to permit stopped flow analysis, but fast enough to preclude particle overlap.
Traditionally, the flow of specimen and sheath fluids has been controlled by using air pressure to drive these fluids to and through the flow cell. The air pressure applied to each fluid can be adjusted to change the relative flow rate of that fluid through the flow cell. However, such adjustments must be made empirically to obtain the best image capture results, since many other factors affect the flow rate of each fluid as well (e.g. fluid viscosity, flow path resistance, etc.) which can vary system to system, and from fluid to fluid. Moreover, it has been found that flow cell performance can be highly sensitive to the position (translational and angular) of the needle or cannula used to inject the specimen fluid into the flow cell. Thus, it can be a time consuming task to manufacture, test and/or operate the flow cell with optimal and reliable performance.
The present invention is a flow cell for examining specimen fluid flowing with sheath fluid. The flow cell includes a housing defining a hollow fluid passage that has an injection point, a geometric focusing portion in which the fluid passage narrows in a cross section dimension thereof, and an examination area, a cannula having an output end disposed at the injection point in the fluid passage, a first direct flow control pump for pumping the sheath fluid through the fluid passage such that the sheath fluid has a first known velocity at the injection point, a second direct flow control pump for pumping the specimen fluid through the cannula such that the specimen fluid is injected into the fluid passage by the cannula output end as a stream of the specimen fluid having a second known velocity at the injection point wherein the second known velocity is different from the first known velocity, and a measurement device for measuring a parameter of the specimen fluid stream passing through the examination area. A cross section dimension of the specimen fluid stream is focused by the sheath fluid via linear flow rate focusing and by the narrowing geometric focusing portion of the fluid passage via geometric focusing.
Another aspect of the present invention is a method of flowing specimen fluid and sheath fluid through a hollow fluid passage of a flow cell having an injection point, a geometric focusing portion in which the fluid passage narrows in a cross section dimension thereof, and an examination area. The method includes flowing the sheath fluid through the fluid passage such that the sheath fluid has a first known velocity at the injection point, injecting the specimen fluid into the fluid passage at the injection point as a stream of the specimen fluid having a second known velocity wherein the second known velocity is different from the first known velocity, and measuring a parameter of the specimen fluid stream passing through the examination area. A cross section dimension of the specimen fluid stream is focused by the sheath fluid via linear flow rate focusing and by the narrowing geometric focusing portion of the fluid passage via geometric focusing.
One more aspect of the present invention is a method of forming a cannula from a hollow tube having first and second ends and a first cross-section shape. The method includes cutting a hollow tube to a desired shape wherein the cut tube has a first end and a second end and a first cross-section shape, inserting a first mandrel having a first thickness into the first end, crushing the first end onto the first mandrel, removing the first mandrel from the first end, and then inserting a second mandrel having a second thickness into the first end wherein the second thickness is less than the first thickness, crushing the first end onto the second mandrel, and removing the second mandrel from the first end. After the crushings of the first end, the first end has a second cross-section shape that is different from the first cross-section shape.
Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures.